Messages in this thread | | | Date | Mon, 9 Jul 2007 16:08:37 -0700 (PDT) | From | Christoph Lameter <> | Subject | Re: [patch 00/10] [RFC] SLUB patches for more functionality, performance and maintenance |
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On Mon, 9 Jul 2007, Mathieu Desnoyers wrote:
> > > Yep, I volountarily used the variant without lock prefix because the > > > data is per cpu and I disable preemption. > > > > local_cmpxchg generates this? > > > > Yes.
Does not work here. If I use
static void __always_inline *slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node, void *addr) { void **object; struct kmem_cache_cpu *c;
preempt_disable(); c = get_cpu_slab(s, smp_processor_id()); redo: object = c->freelist; if (unlikely(!object || !node_match(c, node))) return __slab_alloc(s, gfpflags, node, addr, c);
if (cmpxchg_local(&c->freelist, object, object[c->offset]) != object) goto redo;
preempt_enable(); if (unlikely((gfpflags & __GFP_ZERO))) memset(object, 0, c->objsize);
return object; }
Then the code will include a lock prefix:
3270: 48 8b 1a mov (%rdx),%rbx 3273: 48 85 db test %rbx,%rbx 3276: 74 23 je 329b <kmem_cache_alloc+0x4b> 3278: 8b 42 14 mov 0x14(%rdx),%eax 327b: 4c 8b 0c c3 mov (%rbx,%rax,8),%r9 327f: 48 89 d8 mov %rbx,%rax 3282: f0 4c 0f b1 0a lock cmpxchg %r9,(%rdx) 3287: 48 39 c3 cmp %rax,%rbx 328a: 75 e4 jne 3270 <kmem_cache_alloc+0x20> 328c: 66 85 f6 test %si,%si 328f: 78 19 js 32aa <kmem_cache_alloc+0x5a> 3291: 48 89 d8 mov %rbx,%rax 3294: 48 83 c4 08 add $0x8,%rsp 3298: 5b pop %rbx 3299: c9 leaveq 329a: c3 retq
> What applies to local_inc, given as example in the local_ops.txt > document, applies integrally to local_cmpxchg. And I would say that > local_cmpxchg is by far the cheapest locking mechanism I have found, and > use today, for my kernel tracer. The idea emerged from my need to trace > every execution context, including NMIs, while still providing good > performances. local_cmpxchg was the perfect fit; that's why I deployed > it in local.h in each and every architecture.
Great idea. The SLUB allocator may be able to use your idea to improve both the alloc and free path.
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